DIPTERA. 203 



way" list. Such a family is the family of the robber flies. 

 They are entirely predaceous, fiercely so; and as long as 

 they prey upon injurious insects, all well and good; but 

 when they pounce upon a bee, a tiger beetle, or a dragon 

 fly they are decidedly injurious. They also eat many 

 smaller flies, and the chances are that such flies are better 

 dead than alive. The robber flies are swiftly-flying 

 insects, pointed as to the abdomen; the legs are long and 

 strong and the claws are prominent and set nearly at a 

 right angle with the tarsi. The long-legged flies are 

 much slenderer flies, with long dangling legs which get 

 tangled up as their owners fly uncertainly short distances, 

 and sit down frequently, as if tired of managing such 

 difficult things. They eat smaller flies and gnats. 



Fig. 86. — Aggressive mimicry. On the left, a bee, Bombus mastrucatus; on the 

 right, a bee fly, Volucella bombylans. {Natural size. Folsom.) 



The wasp flies are exceedingly like wasps in body 

 form and coloring, but possess only two wings. They are 

 all flower poUenators, and in addition to this their larvas 

 are parasitic, thus conferring a second benefit. 



The soldier flies have slender legs, often variously 

 shaped, some having the femora and the tibise flattened 

 or spinose or scaly. Most of them look hump-backed 

 because of the thorax' being very convex on the upper 

 side, the neck, where it joins the head to the thorax. 



